The automotive industry has made very significant progress in reducing automotive emissions. Tighter emission standards and significant innovations in catalyst formulations and engine controls have led to dramatic improvements in the low emission performance and robustness of gasoline and diesel engine systems. This has reduced the environmental differential between optimized traditional fuel systems and alternative fuel systems for automotive vehicles. However, many technical challenges remain to make the internal combustion engine a nearly zero emission system having the efficiency necessary to make the vehicle commercially viable.
The emergence of fuel cell technology as a potential automotive power system has led to interest in producing syngas from transportation fuels such as gasoline and diesel fuels for use in the fuel cell. Syngas is a gaseous fuel containing primarily hydrogen and carbon monoxide. A fuel cell is an energy conversion device that generates electricity and heat by electrochemically combining a gaseous fuel, such as syngas, and an oxidant, such as air or oxygen, across an ion-conducting electrolyte. The fuel cell converts chemical energy into electrical energy. Fuel cells generally consist of at least two electrodes positioned on opposite sides of an electrolyte. The oxidant passes over the oxygen electrode (cathode) while the fuel passes over the fuel electrode (anode), generating electricity, water, and heat. The fuel stream that is supplied to the anode includes hydrogen. For example, the fuel stream may be a gas such as substantially pure hydrogen or a reformate stream containing hydrogen. The oxidant stream, which is supplied to the cathode, comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air.
Solid oxide fuel cells (SOFC) are constructed entirely of solid-state materials, utilizing an ion conductive oxide ceramic as the electrolyte. A fuel cell stack commonly includes numerous inlet ports and supply manifolds for directing the fuel and oxidant to a plurality of anodes and cathodes respectively. The stack often also includes an inlet port and manifold for directing a coolant fluid to interior passages within the stack to absorb heat generated by the exothermic reaction in the fuel cells. The stack also generally includes exhaust manifolds and outlet ports for expelling the unreacted fuel and oxidant gases, as well as an exhaust manifold and outlet port for the coolant stream exiting the stack.
In a typical SOFC, a fuel flows to the anode where it is oxidized by oxide ions from the electrolyte producing electrons that are released to the external circuit. The reaction also produces water and carbon dioxide. At the cathode, the oxidant accepts electrons from the external circuit to form oxide ions. The oxide ions migrate across the electrolyte to the anode. Each individual electrochemical cell generates a relatively small voltage. The flow of electrons through the external circuit provides for consumable or storable electrical power. Higher voltages are attained by electrically connecting a plurality of electrochemical cells in series to form a stack.
An SOFC can be used in conjunction with a reformer system that catalytically converts a hydrocarbon fuel to a reformate stream of hydrogen and carbon monoxide. The reformate stream is then fed into the fuel cell to generate electricity for use or storage. Different types of reformer technologies include steam reformers, dry reformers, and partial oxidation reformers.
Steam reforming systems involve the use of a fuel and steam mixture that is reacted in heated tubes filled with catalysts to convert the fuel into principally hydrogen and carbon monoxide. The steam reforming reactions are endothermic, thus the steam reforming systems are designed to transfer heat into the catalytic process. An example of a steam reforming reaction is illustrated in Equation I.CH4+H2O→CO+3H2  I.
Partial oxidation reformers are based on substoichiometric combustion to achieve the temperature necessary to reform the fuel. Decomposition of the fuel to primarily hydrogen and carbon monoxide occurs through thermal reactions at temperatures ranging from about 700° C. to about 1,000° C. Catalysts have been used with partial oxidation systems to promote conversion of various sulfur-free fuels, such as ethanol, into synthetic gas. The use of a catalyst accelerates the reforming reaction and lowers the reaction temperature required. An example of a partial oxidation reforming reaction is shown in Equation II.CH4+½O2→CO+2H2  II.
Dry reforming involves the creation of hydrogen and carbon monoxide in the absence of water, for example, using carbon dioxide as the oxidant. Dry reforming reactions, like steam reforming reactions, are endothermic processes. An example of a dry reforming reaction is depicted in Equation III.CH4+CO2→2CO+2H2  III.
Practical reformers may use a combination of these processes. By using a combination of these processes, the temperature of the reforming process can be manipulated. For example, increasing the amount of air or decreasing the amount of endothermic oxidants, i.e., water, carbon dioxide, supplied to the process, will increase the reformer temperature.
On-board reforming of gasoline and diesel fuels is difficult because of the wide variety of fuel components, additives and contaminants found in these fuels. These fuels are difficult to fully vaporize and contain compounds that readily form soot and other carbonaceous matter as well as sulfur bearing matter. The catalysts used in the reformer systems are sensitive to sooting and contamination. When sooting occurs, the active catalyst material can be fully or partially deactivated adversely affecting the efficiency and operating lifetimes of the reformer system. Moreover, the soot, formed primarily from carbon, can react exothermically when exposed to an oxidant such as air. A rapid increase in temperature may cause melting and destruction of the catalyst support as well as a loss of the precious metal material that makes up the catalyst material.
Moreover, since rapid start-up and shut down cycles are typical in automobile applications, the reformer catalyst bed must be heated rapidly. However, the air/fuel mixing zone must be carefully controlled in temperature to avoid gas phase reactions in the mixing zone of the reformer system. Particularly when using reactive fuels like gasoline or diesel fuels, gas phase reactions tend to produce lower quality reformate having more soot, and less hydrogen. This proves to be inefficient because soot build-up can rapidly impair reformer performance as well as components disposed downstream.